There is now increasing evidence that the differentiation of CD4+ T cells into regulatory T cells (Tregs) constitutes a significant barrier to effective host anti-tumor immunity and contributes to the establishment of tumor tolerance. In spite of this, we have demonstrated that lymphocytes from "tolerant" tumor-bearing donors paradoxically elicit transient effector function upon transfer into irradiated, tumor-bearing recipients undergoing syngenic bone marrow transplantation. If this response is sub-curative however, tolerance is reestablished with tumor relapse. The central hypothesis of this proposal is that changes observed in the function of tumor specific T cells during immune reconstitution are tightly controlled the frequency and/or function of such regulatory T cells. We will examine strategies to manipulate the balance of effector and Treg repopulation in the post-transplant period to augment anti-tumor immunity. Specifically, we will: 1. Track the frequency and function of "natural" and tumor induced regulatory T cells (TMTregs) present in a lymphocyte graft from tumor-bearing donors as they repopulate the transplant recipient during immune reconstitution. We will examine the influence of depleting subpopulations expressing candidate TMTreg associated antigens from the graft on T cell effector function, response to vaccination, and relapse rates. 2. Examine the frequencies of TMTregs and effector cells during antigen specific and/or anti-CD3/CD28 driven in vitro polyclonal T cell expansion performed for adoptive therapy. Depletion of the subpopulations identified in Aim 1 prior to ex vivo expansion will be tested as a means of promoting sustained effector responses during immune reconstitution. 3. Seek to reduce the de novo generation of TMTregs in the post-transplant period by systemic administration of agents targeting Tregs in vivo, modulating antigen presenting cell function (TLR agonists, STAT-3 inhibition), or blocking cytokine dependent effects (e.g.anti-IL10, anti-TGF-D). 4. Perform clinical trials of Treg depleted, ex vivo expanded autologbus T cells in lymphodepleted patients (pts) with hematologic malignancies. Follow-up studies will examine the integration of cancer vaccines and strategies of systemic modulation of immunity that show activity in the preclinical studies, performed in aim 3.